A photochemical macrocyclization route to asymmetric strained [3.2] paracyclophanes.
The intricate frameworks of paracyclophanes found in various chiral auxiliars, solar cells, high-performance plastics, pharmaceuticals, and molecular machines have become an important target for synthesis. Whereas numerous methods exist for the preparation of symmetric paracyclophanes, protocols for the efficient synthesis of strained asymmetric scaffolds are limited. Here we report a remarkable simple photochemical route to strained [3.2]paracyclophanes starting from readily available educts. By way of NMR and x-ray analyses, we discovered that UV-irradiation of an aromatic carboxylic ester tethered to a toluene moiety leads to the intramolecular formation of a new C-C bond, with loss of an alcohol. A systematic evaluation of the reaction conditions and substituents, as well as radical starter and triplet quenching experiments suggest a reaction mechanism involving an excited triplet state and hydrogen atom transfer (HAT). The robust and versatile new method succeeded in the synthesis of range of cyclophanes with different substitutions, including an unusual diastereoisomer with two planar chiral centers and thus proved to be a valuable addition for the synthetic toolbox.